In constant displacement compressors it is often desirable to provide a variable output. One approach has been the use of a variable speed motor to drive a constant displacement compressor. Another approach has been the unloading of one or more cylinders as by keeping the suction valve unseated during the compression stroke. Such an arrangement is complex, costly and requires pneumatic or hydraulic power elements. While these approaches work, their use has certain inherent disadvantages. When a discretely variable speed motor is used it is generally necessary to shut down the system in order to change the speed and it is necessary to keep the system off for a short period of time in order to avoid restarting against the discharge pressure. When an infinitely variable speed motor is used, an inverter is required with resultant energy loss, etc. Unloading the cylinder(s) often does not provide sufficient flexibility of operation. In a conventional single speed, two-cylinder compressor, the unloading of one cylinder gives you a choice of 100% or 50% of capacity. The structure necessary to keep the suction valves unseated often presents problems due to the need to locate the structure in or on the casing and the resultant requirement for support structure which is usually in excess of that ordinarily provided and, in the case of hermetic compressors, often requires unavailable space.